Symposium Conference Proceedings

Underfill and Flux Compatibility from a Curing Perspective

Authors: Kevin Tan, Juan Sun, Pong Yew Kong, Casey Ng
Company: Henkel Corporation and Nanyang Technological University
Date Published: 4/18/2012   Conference: Symposium

Abstract: The use of underfill to reinforce solder bumps in chipscale packages (CSP) and flipchips (FC) have been well documented over the years. The underfill acts as a buffer to compensate for the difference in Coefficient of Thermal Expansion (CTE) between the solder joint and the substrate and enhances overall reliability. This enhancement, however, is only achieved if there is adequate compatibility between the underfill material and the solder flux residue.

The flip chip process requires the use of solder flux to remove the oxide layers from the bond pads and allow the solder joint to form. In addition, the flux also acts as a protective layer that prevents subsequent oxidation after the joint formation. Traditionally, water soluble solder paste is used and the flux residues after reflow are washed off before the underfill process. The miniaturization of electronics devices and consequent reduction in gap height, in addition to the lead free regulation have made the cleaning process very challenging in recent years. In addition, many manufacturers are moving towards the no-clean solder paste technology where flux residues are left on the board after the reflow process. In this process, there is direct contact between the flux residue and the underfill. Hence, flux compatibility is becoming an increasing important criterion for materials selection but it is puzzling that many manufacturers still do not see the importance of this aspect. In fact, there is a wide selection of underfill chemistries in the market today.

Flux compatibility can be classified in two ways, namely chemical flux compatibility and physical flux compatibility. Physical flux compatibility refers to the flux residue effect on the flow ability and the substrate wettabilty of the underfill. This area has been well studied by Jinlin Wang from Intel Corporation using quartz dies[1]. On the contrary, the field of chemical flux compatibility, which is compatibility from the curing perspective, is relatively less studied. Previous attempts to study chemical compatibility are largely limited to the anhydride chemistry underfill[2]. In the industry, the wide variety of underfill chemistries used include the anhydride underfill, cyanate ester underfill, cationic underfill, amine underfill, phenol underfill as well as the catalyst assisted amine underfill. Each of the different chemistry has its pros and cons and the selection of a particular candidate really depends on the application itself, for example, the cationic cure underfill has a latent heat catalyst that enables the user to cure the underfill at very rapid rate at a low temperature. A selection of six underfills was studied in relation to three halide free and one halogen free solder paste for chemical compatibility.

Key Words: 

Underfill, flux, curing

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